Unfortunately you're never going to stop people stealing your intellectual property if they REALLY want it. If they can see it (on a web page), they can steal it - as Gobae has demonstrated! You'll see the big stock photo sites only let you see the images in lower resolution and watermarked. They have the money and resources and this is all they do!
Preventing theft can become an obsession. Trust me, its not worth the lost sleep. If you come across someone using your images on the web, rather than getting your lawyer write a cease and desist letter, you could make them an offer to continue using the image. It's usually a lazy web designer who stole the image in the first place, they'd probably rather pay you a few bucks than have to go looking for a replacement image!!!